I've got a 1994 BMW 325iC that won't pass smog (for more details, see this thread). I took it to a shop to get both a leak-down and compression test done with some interesting numbers:
Cylinder 1 2 3 4 5 6 Compression 180 210 160 210 205 210 Leak-Down 18% 15% 40% 14% 18% 15%
Obviously it's not holding compression very well across the board, and cylinder 3 is especially leaky. Compression numbers indicate both cylinders 1 and 3 are struggling a bit too.
I was informed by the shop that a quick look into the cylinder revealed quite a bit of carbon buildup, which I can assume is not only on the piston head, but also on the valves and valve seats, which both the shop and I suspect is the primary cause of the leak.
After using some intake manifold cleaning aerosol and dumping nearly two gallons of isopropanol into the tank (shop suggestion, and I trust them), the only truly obvious result is that my idle no longer surges.
Previously, when the engine reached operating temperature, the idle would steadily climb to ~1100RPM over about a half-second, pause for a half-second, then suddenly drop to ~500RPM before repeating the cycle.
Now, it starts up (even when warm) between 600 and 750RPM. After driving for a bit, it will stabilize to ~950RPM, but without surging.
I would like to believe some of the carbon buildup has been removed by my efforts to chemically clean the engine, but I don't know if the elimination of surging is a good indicator of this, which is why I am asking if carbon buildup on the valve seats can cause the engine idle to surge.